An optical tracking system may be used for tracking a target. Recently, in order to process precise surgery while minimizing a risk of the occurrence of surgical errors, a method has been used that tracks the location (or coordinates) and the posture (or orientation) of a surgical robot or a surgical instrument and utilizes the tracking result for surgery. The location of a target, for example, may be defined as spatial coordinates, such as coordinates on the X, Y, and Z axes of an orthogonal coordinate system. The posture of a target may be defined as a roll, pitch, or yaw. In order to accurately track a target, it is important to accurately recognize the location and the posture of the target, which correspond to six degrees of freedom as described above.
In the optical tracking system, for example, after attaching a reference body, such as a marker, to the target, the marker is tracked in order to determine the location and the posture of the target. For example, the location and the posture of a target may be measured by attaching to the target a structure, to which three or more markers are attached. The method of using three or more markers may cause a problem in which it is difficult to reduce the size of the tracking reference body. In particular, when a plurality of markers is attached to a tool, such as a surgical instrument, which is directly navigated by a user, the user may be interrupted or inconvenienced due to its size and weight while using the instrument.